1. Field of the Invention
The present invention relates to a process for preparing a toner for developing an electrostatic image formed by a process such as electrophotography, electrostatic recording or electrostatic printing.
2. Related Background Art
As disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-2478, there are large number of electrophotographic methods. In general, copies are obtained by forming an electrostatic latent image on a photosensitive made of photoconductive material. Then the latent image is developed by the use of a toner and the toner image is transferred to a transfer medium such as paper if desired, after which the toner image is fixed by the action of heat, pressure, heat-and-pressure, or solvent vapor.
Toners are required to have a sharp particle size distribution. In the process of producing a toner, coarse particles that may adversely affect image quality or fine particles that may cause fog are removed by providing classification steps.
In the classification process fine particles of not more than 2 to 3.mu. firmly adhere electrostatically to particles having the desired particle size and such particles are difficult to separate. These fine particles firmly adhere to the surface of each part of the developing unit and are fixed there, tending to cause ghosts or a deterioration of images and a lowering of density when copies are taken in a large number. As a means for solving such problems, Japanese Patent Application Laid-Open No. 53-58244 proposes a method in which a fine silica powder is added to a colored resin powder that serves as a toner, which are mixed and then classified into powder with a specific particle diameter, or, after classification, further heated to carry out a treatment for making the particles in the powder spherical.
The method disclosed in the above Japanese Patent Application Laid-Open No. 53-58244 employs a V-type mixer when silica powder is mixed with toner. The dispersion power of the V-type mixer is relatively weak, so that agglomerates tend to be present in a toner. Consequently, white dots tend to appear at a black solid area of a toner image, and fog or the like tends to appear in the non-image area. This method has an additional problem in that the amount of silica powder may change from the amount when added. This problem is due to the variability of mixing conditions, the types of classifiers employed as well as the classification conditions.
In general, toners are prepared by melt-kneading at least a resin and a coloring agent and other additives, followed by pulverization and classification to control the particle size of the resulting powder. In the course of the classification, powder is removed as coarse powder or fine powder in an amount of from 15 to 40% by weight based on the feed. The amount of powder removed depends on the quality required for toners or the performance of a classifier used. For economy, the coarse and fine powders which were removed are blended with starting materials at the time of melt-kneading.
In the above method proposed in Japanese Patent Application Laid-Open No. 53-58244, the powdery silica and additives which originally should not be included in toner particles, are mixed into the coarse powder or fine powder at the time of the classification. The resulting coarse and fine powders are difficult to recycle because the powders contain silica powder and additives.
When the powdery silica and additives are mixed with a pulverized product in the presence of a large quantity of the fine powder, the various substituents are not well dispersed. This is because the fluidity or agglomerating properties of the powdery silica are higher than those of a toner. As a result, removal of the fine powder during classification as well as the quality problems noted above can not be eliminated.
A conventional process for producing a toner will be further detailed with reference to the accompanying FIGS. 2 and 3.
FIGS. 2 and 3 show flow charts of the respective steps in conventional processes for producing toners.
The conventional process as shown in FIG. 2 can achieve a superior utilization efficiency of starting materials, but tends to result in an insufficient removal of fine powder (in particular, the one with a particle size of not larger than 2 to 3.mu. as described above). This process has a limit in the removal of the fine powder even if the amount of powder discharged to the fine powder side is increased at the time of classification. Hence, not only the problems in quality as previously discussed are brought about, but also an increase in cost tends to be caused because of an increase in the amount of recycling into the kneading step.
The toner production steps as shown in FIG. 3 correspond to those of the production process disclosed in the Japanese Patent Application Laid-Open No. 53-58244. The process shown in FIG. 3 can achieve more effective removal of the fine particles of not larger than 2 to 3.mu. or less as compared to the process shown in FIG. 2. However, as previously discussed, the removal of the fine particles of not larger than 2 to 3.mu. is still unsatisfactory. In addition, the fine powder in which silica is included is difficult to be recycled which causes an increase in cost of toners.